Use of the working material as part of the crystal making apparatus



J1me 1965 c. R. MORELOCK 3,

USE OF THE WORKING MATERIAL AS PART OF THE CRYSTAL MAKING APPARATUSFiled May 51, 1962 7'0 Vacuum I I 2a W 29M 56 f I I/4Z g 4, 1 3 n 43 LV2/ o I I Q O 54 O I o 53 o ,+-20

lnvenfor: Char/es R More/oak,

His Afforney' United States Patent 7 F 3,188,182 USE OF THE WORKKNGMATERIAL AS PART 025 THE CRYSTAL MAKING APPARATUS Charles R. Moreiock,Ballston Spa, N.Y., assignorv to General Electric (Iompany, acorporation of New York Filed May 31, 1962, Ser. No. 2%,023 3 Claims.(Cl. 223-273) This application is a continuation-in-part of applicantscopending application, Serial No. 120,560, filed June 29, 1961, nowabandoned, and assigned to the same assignee as the present invention.

This invention relates to metal single crystals and more particularly toimproved processes and apparatus for producing such crystals.

Prior to this invention, the production of vapor-deposited singlecrystals has been limited to those materials having low melting pointsand relatively high oxide dissociation pressures. The greatly improvedphysical properties offered by single crystals, as compared topolycrystalline materials, have not been attainable due to the absenceof processes for etfectively producing such crystals.

ildhdd Patented June 8, 1955 ture of the growth substrate body belowthat of the source It is a principal object of this invention to providean improved process for producing metal singlecrystals or whiskers.

Another object of this invention is to provide an improved process forproducing single crystals of metals having low oxide dissociationpressures.

An additional object of this invention is to provide improved apparatusfor producing metal single crystals.

Other objects and advantages of the present invention will be in partobvious and in part explained by reference to the accompanyingspecification and drawings.

In the drawings: 7

FIG. 1 is a side elevation, partly in section, showing an apparatus forproducing metal crystals according to the present invention;

FIG. 2 is a perspective showing a combined source metal and substrategrowth body for use in the the apparatus of FIG. 1;

FIG. 3 is a perspective of another form of source metalgrowth substratecombination; and

FIG. 4 is yet another form of source metal and growth substratecombination for growing single crystals.

The process of this invention generally comprises 10- eating a sourcemetal body, which may be made of a metal suchas chromium, copper, gold,nickel, iron, cobalt, or binary or ternary alloys of iron, nickel andcobalt in a chamber isolated from the outside atmosphere, which chambercan be heated to a temperature sufiicient to vaporize the source metal.A growth substrate body which may be compositionally identical to thesource metal, or of a different composition if desired, is also locatedWithin the vaporizing chamber that is maintained at a temperature lowerthan that of the source metal body. As the chamber is heated to atemperature sufiicient to vaporize the source metal, the chamber iscontinuously evacuated so that some portion of the source metal vapor inthe chamber escapes toward the outlet and getters oxygen from' thechamber to maintain the oxygen pressure therein below the dissociationpressure of the source metal oxide.

The apparatus of this invention generally comprises means defining avaporizing chamber, such asa fused silica tube, evacuating meansoperably connected to the vaporizing chamber so that the atmospherewithin the chamber can be continuously removed, a body of source metaland a growth substrate body located within the metal body.

The apparatus of the invention can be described more completely byreferring to FIG. 1 of the drawings. In this figure, the numeral 19indicates the vaporizing chamber, this chamber being defined by a fusedsilica tube 11 and surrounded by heating means, here shown as a platimum-wound furnace 12. The furnace 12 surrounds only a portion of theentire length of tube 11 so that the heating zone where vaporization anddeposition of metal single crystals is carried out is not coextensivewith the entire length of tube 11. A Pyrex tube 15 has been integrallyjoined to the upper end of tube 11 and the upper end of this tube isconnected to a source of vacuum, not shown. Extending outwardly from butintegrally joined to the tube 15 is a lateral extension 16 which iscompletely sealed so that no opening is present into the volume definedby tubes 11 and 15. The Pyrex glass was substituted for quartz in themanner described because it is easier to work and shape and is cheaperin the overall construction of the apparatus.

A generally cylindrical source metal body 2 9 is shown positioned withinthe hot zone of vaporizing chamber 16, the body 20 having, of course, anaxial opening extending I through it. A growth substrate body is locatedapproximately equidistant from the inner surface of body 2t) and extendsupwardly beyond the upper limit of body 20 a distance sufficient topermit a portion of its length to be located outside of the hot zonedefined by furnace 12.

The growth substrate body 21 can be lowered into the operating positionshown and, conversely, raised completely out of the hot zone defined byfurnace 12 bymeans of the winch mechanism indicated generally by numeral.25. This mechanism comprises a tungsten wire 26 which can be wound andunwound onto reel 27. The reel 27 is fixedly attached to rotatable shaft28 mounted in journal 29. That end of shaft 28 furthest removed fromreel 27 has a magnet 30 connected to it, which magnet can be operated bythe rotatable operating member 31. Member 31 comprises a body portion 32constructed of any suitable plastic or non-conducting material andcontains a plurality of magnets 33 which cooperate with the magnet 30 torotate shaft 28 as member 31 is turned. As shown in the figure,clockwise rotation of member 31 will wind fiber 26 on reel 27 and raisesubstrate body 7 21 out of operating position within the vaporizingchamber 10.

Generally speaking, when the various elements of the apparatus are inthe positions indicated in FIG. 1, metal single crystals, or whiskers,can be grown by operating furnace 12 to raise the temperature withinvaporizing chamber 10 to a point sufiicient to effect vaporization of pthe source metal body 20. Since a portion of the growth radiatedoutwardly from that portion of body 21 which is located beyond thelimits of the hot zone defined by furnace 12. Thus, metal vapor presentin chamber 10 will deposit as single crystals. 2

It is felt that the single crystals or whiskers nucleate V on emergentscrew dislocations located on or in the surface of the substrate body.Once started by this initial nucleation, the screw dislocation becomesbuilt into the crystal and axial growth proceeds by metal atoms beingc1) adsorbed on the sides of the crystal, migrating to the tip andbecoming incorporated in the lattice at the step of the screwdislocation.

According to this mechanism, crystals should grow at supersaturationratios which are less than some critical supersaturation ratio, ocrequired for measurable twodimensional homogenous nucleation. Thesupersaturation ratio, a, is equal to p/p where p-is the actual pressureof-the depositing-vapor and p the saturation pressure of the depositedvapor. These pressures are established by the temperatures of theevaporation and growth sub stratesurfaces.

According to the proposed growth mechanism, there should be no lowervapor pressure limit for the occurrence of-single crystalgrowth. For themetals with which this invention is particularly concerned, viz.,chromium, copper, gold, nickel, iron, cobalt, or binary or ternaryalloys of iron, nickel and cobalt, no single crystals were grown atvapor pressures less than mm. of mercury. It is probable that thegettering action of the metal vapor, made possible by continuousevacuation of the vaporiz ing chamber, is sufficient to keep the oxygenpressure in the growth zone below the dissociation pressure of the metaloxide only at metal vapor pressures of 10 or greater. The dissociationpressures for the oxides of chromium, iron, cobalt, nickel and copperare approxi mately 10- 1O 10* 10' and 10* mm. of mercury, respectively.Since in all the experiments conducted the metal vapor pressures wereall nearly the same, it would be expected that oxides in the chromiumexperiments would be more severe than the copper experiments. This wasobserved to be true.

It is therefore important in carrying out the present processthat'before the condition of saturation ratio is established, thepartial pressure of oxygen must be below the dissociation pressure ofthe particular metal oxide if whisker growth is to occur. The uppervapor pressure limit should only be p at the melting point, T of themetal being investigated. The closest approach to this limitation hasbeen in experiments with gold and copper, whiskers "having been grownwhen the substrate temperature was held within C. of their respectivemelting points.

Whisker growths, then, are not restricted .to any particular magnitudeof the metal vapor pressures but rather to the upper and lower limitsdetermined as above. This is substantiated by the fact that the vaporpressures used in whisker growth investigations have ranged from 10* mm.of mercury for gold, to 0.1 mm. of mercury for cadmium.

' Several experiments were run to grow metal single crystals or whiskersof various types of metals. In these experiments, growth substratebodies and source metal bodies were positioned in the vaporizing chamberas shown in FIG. 1 of the drawings while the chamber was beingheatedcontinuously by furnace 12 and was also beingcontinuously evacuated.Inthese experiments, the apparatus was heated to operating temperatureprior to the lowering of the growth substrate body into the vaporizingchamber so that any oxygen or other atmospheric contaminants intheapparatus were removed as low as possible. The useful ranges of theevaporation temperatures'for the various materials are listed in thefollowing Table I.

Table I Evaporation temperature Metal: Range, C. Chromium 830l890 Nickel10804455 'Gold 700-1063 Copper 820-1083 Iron 1050-1539 Cobalt 1000-149540 in FIG. 3 of the drawings. -metal is a flat sheet 41 which isconnected to a larger The source metal, evaporation temperature, growthtime-whisker length and the composition of the growth substrate forthese various samples are all listed in Table 11 following:

Table 11 l l Evap. Growth Longest Growth Evap. Metal Temp. Time WhiskersSubstrate C.) '(Hrs.)

1, 050 16 70 Mo Screen. l

900 None Do. 1, 200 16 100 D0. 1, 050 48 None Do. 1, 050 16 300 AuSheet,

875 120 150 Do. 1 830 70 None On Sheet. 930 16 200 Do. i 1, 020 1 NoneD0. 1, 000 3 100 Do. 1 1,000 16 100 Do. l 950 50 300 D0. j 1, 030 16None Fe Sheet 1 1,060 16 2 Do. 1 1,150 None Mo Screen 1 1,150 1 15 Do.1, 060 so 200 Fe Sheet. i 1,150 16 100 D0. Co 1,020 $6 5 Mo Sheet. 1 Co(23%)Fe 77%) 1,120 50 Do. i

As mentioned previously, it isnecessary that the growth 3 substratebodybe maintained at a temperature below that i of the source metal bodyif crystal growth is to be obtained. This temperature diiterential canbe obtained in any one of several different ways. For example, in FIG. 2of the drawings, the growth substrate body and source 1 metal body existas'a single unit, here shown as a spirallyi wound coil 35. 'In thiscase, metal is evaporated from i the inner surface of theoutermost'spiral' of coil 35, since i it is subjected directly-toradiant heating from the furnace and deposited on the outermost surfaceof the second spiral 37 since this surface is protected from directheating of the furnace and is therefore somewhat cooler. This same-vaporizing and'depositing process takes place on each successivespiral, progressing inwardly and ending at the innermost spiral 38. Ithas been found that the normal temperature differential existing betweensuc- 1 cessive spirals is about 17 C., temperature differentials on Ithe order of 15 and more normally being sufficient to deposit vaporizedmetal for whisker growth.

A slightly modified combined form of source metal body and growthsubstrate body is indicated by numeral In this case, the source growthsubstrate body 42 by means of connecting pieces 4-3. The body 42 issomewhat longer than body 41 so that it can extend-upwardly out of thehot zone defined by the heating .furnace in much the ame manner as thegrowth substrate body 21 shown in FIG. 1 of the drawportion locatedoutside of the hot zone.

tion which is particularly useful in the present invention.

In this instance, the source metal body is shown as a wire 50 whichextends down through the center of a completely closed growth substratebody 51. The wire 50 is insulated from body 51 by means of an insulatinggrommet 52 but is electrically connected to the bot-tom plate of body51- so that electricity can be made to flow through both body 50 andbody 51. The electricity is supplied from a source 53 via the wires 54.The operation of this device is eifected by placing the structure in thevaporizing zone, such as shown in FIG. 1, and heating. the assembly to atemperature below that Where any metal condensation will occur. By thenconducting a flow of electricity through the two bodies, the temperatureof Wire 50 can be raised beyond the temperature of body 51. Thistemperature differential results from the carrying capacity of the twobodies. That is, the relatively small size of wire 50 results in aresistance greater to the passage of electricity than is present in body51, so that electrical heating of wire 50 is effected beyond that whichtakes place in body 51. This apparatus makes it possible to accuratelycontrol the temperature differential between the growth substrate bodyand the source metal body.

The function of continuously evacuating the vaporizing chamber isimportant to this invention. Since metals having extremely low oxidedissociation pressures are contemplated, it is necessary that any oxygenin the system be removed to a level as low as possible. It is furtheressential that oxygen diffusing into the system through the walls of thevarious vessels be prevented from coming into contact with metal vaporin the growth zone. These objectives are attended by continuouslyevacuating the atmosphere from vaporizing zone in a manner such that acontinuous supply of metal vapor is directed upwardly through tubes 11and 15 to getter out any oxygen which may be present. This operationmakes it possible for the first time to maintain the oxygen pressure inthe vaporizing chamber below the dissociation pressure of the sourcemetal oxides, where the metals being vaporized are extremely strongoxide formers. Of course, the apparatus and process are equallyetfective in growing crystals of those materials where oxygencontamination is a relatively minor problem.

Although the present invention has been described in connection withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An apparatus for growing metal single crystals comprising, meansdefining a vaporizing chamber for growing single crystals of a sourcemetal on a substrate, means operably connected to said vaporizingchamber to elfect continuous evacuation of the atmosphere therefromduring operation, means for heating said source metal body and saidsubstrate body, and means separate from said heating means for renderingthe temperature of said substrate body below that of said source metalbody, said separate means being in part said source metal body.

2. An apparatus for growing metal single crystals comprising, meansdefining a vaporizing chamber for growing single crystals of'a sourcemetal on a substrate, means operatively connected to said vaporizingchamber to effect continuous evacuation of the atmosphere therefromduring operation, means for heating said source metal body and saidsubstrate body, means separate from said heating means for rendering thetemperature of said substrate body below that of said source metal body,and means electrically connecting said source metal body and saidsubstrate body to a source of electricity to provide for a flow ofelectricity through said bodies, to render the temperature of saidsubstrate body below that of said source metal body.

3. An apparatus as defined in claim 1 wherein said substrate bodyincludes a portion extending beyond the hot zone created by said heatingmeans to withdraw heat from a portion located within the hot zone andthereby maintain the portion within the hot zone at a temperature lowerthan that of said source metal body.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESGriest et al., article in the Journal of Applied Physics, vol. 27,September 1956, pages 1022-1024.

Holland, Vacuum Deposition of Thin Films, John Wiley and Sons, Inc.,1956, page 20.

DAVID L. RECK, Primary Examiner.

1. AN APPARATUS FOR GROWING METAL SINGLE CRYSTALS COMPRISING MEANSDEFINING A VAPORIZING CHAMBER FOR GROWING SINGLE CRYSTALS OF A SOURCEMETAL ON A SUBSTRATE, MEANS OPERABLY CONNECTED TO SAID VAPORIZINGCHAMBER TO EFFECT CONTINUOUS EVACUATION OF THE ATMOSPHERE THEREFROMDURING OPERATION, MEANS FOR HEATING SAID SOURCE METAL BODY AND SAIDSUBSTRATE BODY, AND MEANS SEPARATE FROM SAID HEATING MEANS FOR RENDERINGTHE TEMPERATURE OF SAID SUBSTRATE BODY BELOW THAT OF SAID SOURCE METALBODY, SAID SEPARATE MEANS BEING IN PART SAID SOURCE METAL BODY.